Due to the sharp and continuing increase in demand for polycrystalline silicon as source material for silicon for semiconductors and as source material for solar cells, demand for thin silicon rods with a diameter of approx. 4-10 mm is also rising accordingly. Production of thin silicon rods is an essential and sometimes restrictive part of the production of silicon rods by pyrolytic deposition of silicon from trichlorosilane (a so-called Siemens process) or also from monosilane. As the total length of the thin silicon rods corresponds to the length of the Si rods produced, correspondingly large quantities of these thin rods are used with the sharp rise in the production of silicon for solar uses and for semiconductors.
Thin silicon rods are produced using the conventional pedestal method, for example, in which the rod is drawn out of the inner hole in a single-coil induction coil which melts the upper end of a raw rod with a diameter of 30-50 mm at the same time. Various methods have been described to produce silicon rods.
An example is described in DE 10 2005 016 776 A1 in which, in an initial phase, a thin throat is drawn vertically downwards through the central opening in a single-coil induction coil from a suspended melt drop by known means. When the diameter of the crystal to be drawn is suitable for the surface dimensions, rotation of the growing crystal is reduced to a speed of less than 1 rpm. A single Si crystal with an approximately round cross-section is then drawn vertically downwards in a growth phase during which the inductor used has means of producing an almost rotationally-symmetrical temperature profile at the growth phase boundary.
DE 29 52 602 A1 describes a method and a device for producing thin silicon rods. In this method, a thin silicon rod, secured at its upper end, is drawn upwards from a vertical stock rod, which is melted at its upper end by means of an induction coil. In order to stabilise the thin silicon rod as it grows, corresponding stabilisation measures are provided consisting of spring rods between which the thin silicon rod is guided.
DE 24 58 490 A1 describes another method for producing thin rods. In this method, circular bent thin rods for the support matrix are drawn during material deposition by drawing off from the melted point of a thick rod by means of an annular induction coil along a circular path using a motor with a very low gear and a purely rotational movement.
In each case, only one thin rod can be drawn from the melted part of a raw rod in the above-mentioned method.
Square cores can also be used to precipitate polycrystalline Si rods, these being sawn from large Si rods such as strips. Sawn square cores have an uneven, contaminated surface. As a result they are very sensitive to breakage and must be etched and cleaned by wet chemical processes. If they have been sawn from original, precipitated, very pure polysilicon rods they also have an unfavourably high electrical resistance which complicates the start of the deposition process. They are also always somewhat shorter than the rod used, from which they were sawn.